The present invention relates to a method of fabricating structures from several laminations of metal sheets, and more particularly, to a method of fabricating such structures which combines both superplastic forming and diffusion bonding.
For many years it has been known that certain metals, such as titanium, as well as certain metal alloys, exhibit superplasticity within limited temperature ranges and strain rates. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency towards necking. Thus when in a superplastic condition, the metal or metal alloy exhibits low resistance to deformation and may be elongated with controlled thinning. This permits a sheet of such metal to be readily formed against dies to achieve desired shapes. Superplastic forming (SPF) may be performed in conjunction with diffusion bonding (DB). Diffusion bonding refers to metallurgical joining of surfaces of similar or dissimilar metals by holding them in physical contact and applying heat and pressure sufficient to cause commingling of the atoms at the junction. Further details of both SPF and diffusion bonding may be had by way of reference to U.S. Pat. No. 3,934,441 of Hamilton et al. entitled "Controlled Environment Superplastic Forming of Metals" and U.S. Pat. No. 3,927,817 of Hamilton et al. entitled "Method of Making Metallic Sandwich Structures".
Thinning or necking of parts frequently occurs during SPF, particularly where the formed sheets must undergo substantial deformations in draping around dies which have sharp radii and/or extend substantial distances generally normal to the sheets. This can lead to undesirable weak points. If the sheets are made thick enough to provide for a minimum thickness at such points, there may be excess metal in other areas, resulting in a structure which is unnecessarily heavy. Such extra weight is particularly undesirable in aircraft. Furthermore, where the parts are formed of expensive metals such as titanium alloys, extra material leads to high additional costs. Accordingly, it is desirable that a structure formed by SPF have a controlled thickness or gauge throughout its various regions and contours.
An example of a structure that is difficult to form by SPF is an exhaust mixer of a turbofan aircraft engine. Typically it comprises a plurality of axially extending, circumferentially spaced lobes which surround the core engine. The lobes mix the fan air and the hot exhaust gases from the turbine to obtain improved noise suppression and/or engine performance. Representative configurations of such exhaust mixers are disclosed in U.S. Pat. No. 4,077,206 of Ayyagari and U.S. Pat. No. 4,149,375 of Wynosky et al.
The size and configuration of exhaust mixers is approaching the limits of superplasticity of titanium sheet materials necessary to permit such structures to be formed. For example, to obtain the minimum thickness requirements via SPF in an exhaust mixer for a very large turbofan engine, it would be necessary to start with a sheet thickness of approximately 0.375 inches. Because of the substantial radial extension of the lobes, a sheet must be stretched about 300% in fabricating one type of exhaust mixer by SPF. Even if SPF could be utilized to form an exhaust mixer from sheet material of the foregoing thickness, an extreme amount of selective chemical milling would be required after the SPF in order to obtain uniform thickness throughout the formed structure.
Accordingly, it would be desirable to provide an improved SPF method that could be used to fabricate large, extremely contoured structures while controlling the thickness or gauge throughout the structures.